The present invention relates generally to radio-frequency (RF) power amplifiers. More particularly, the invention relates to an RF power amplifier circuit that includes a bias adjustment circuit that causes the output signal of the amplifier to contain an amount of intermodulation distortion that increases linearly (on a log scale) as the output power increases.
An RF power amplifier is a circuit that is capable of receiving an RF input signal and amplifying it to produce an RF output signal. RF power amplifiers are frequently used in communications systems, such as cable-television systems (CATV). They normally include bipolar junction transistors or field-effect transistors as amplifying elements, and, if so, they also invariably include a bias circuit that sets the quiescent operating point of the transistor. (The quiescent operating point of a transistor is the xe2x80x9cpoint on the family of characteristic curves . . . that corresponds to the average electrode voltages or currents in the absence of a signal.xe2x80x9d JOHN MARKUS, ELECTRONICS DICTIONARY 445 (4th ed. 1979). The quiescent operating point of a bipolar junction transistor includes such parameters as quiescent base-emitter current, quiescent base-emitter voltage, quiescent collector-emitter current, and quiescent collector-emitter voltage.)
In such transistor-based power amplifiers, there are tradeoffs between maximizing efficiency and ensuring that the desired amplification characteristics are maintained. The efficiency of an amplifier, or output circuit efficiency, is the amplifier""s signal output power divided by the power supplied to the amplifier by the power supply. The former quantityxe2x80x94the signal output powerxe2x80x94is determined by the gain of the amplifier and the input drive level (or signal input power). The latter quantityxe2x80x94the power supplied to the amplifier by the power supplyxe2x80x94is largely determined by the quiescent operating point of the amplifier. An amplifier with a high quiescent operating point will draw a large amount of power from the power supply, and the amplifier will have a low efficiency. Conversely, an amplifier with a low quiescent operating point will draw a small amount of power from the power supply, and the amplifier will have a high efficiency. In summary, the relationship among efficiency, signal output power, and quiescent operating point is this: the lower the quiescent operating point and the higher the signal output power, the higher the efficiency of the amplifier.
But an amplifier with a low quiescent operating point and a high signal output power will produce (in addition to an amplified RF signal) large intermodulation and harmonic distortion signals. Harmonics, or noise signals having frequencies that are integer multiples of the signal frequency f (i.e., 2f, 3f, etc.) are generated when a pure sinusoidal waveform is clipped. If clipping occurs when more than one signal is simultaneously applied to an amplifier, then intermodulation distortion (hereinafter, xe2x80x9cIMDxe2x80x9d) occurs in addition to harmonic generation.
For amplifiers with less than a one-octave bandwidth, the dominant IMD term is the third-order component. The theoretical power of such third-order IMD products resulting from waveform clipping increases by about three decibels for every one-decibel increase in signal input power, provided that the highest-order term in the nonlinearity of the amplifier is cubic. In other words, if the theoretical power (in decibels) of the third-order IMD is graphed against the signal input power (in decibels), the result is a straight line having a slope of about 3:1. This graph of the third-order IMD versus input power level is referred to hereinafter as the theoretical 3:1 curve.
In practice, however, real amplifiers contain nonlinear processes that cause the third-order IMD produced to deviate from the theoretically-calculated amount. The power of the IMD caused by these nonlinear processes is determined by the vector addition of the components generated by each nonlinear process. Such terms may partially cancel or add, depending on the frequency and phase of the components. As a consequence, the actual third-order IMD does not follow the theoretical 3:1 curve. As the output power increases, rather, the third-order IMD tends to deviate more and more from the theoretically-predicted 3:1 curve, and the rate of increase of IMD with input power may become 4:1, 5:1 or even worse.
The inventor of the present invention has recognized that this unpredictable variation in IMD from the theoretically-predicted 3:1 curve causes problems to designers of CATV systems, who must be able to calculate signal power and distortion power at each point in a cascaded CATV system. The calculations are normally performed using large spreadsheet programs making use of the assumption that the IMD produced by each amplifier follows the theoretical 3:1 curve. But the calculations are inaccurate if the third-order IMD produced is greater or less than the amount that is theoretically predicted. Thus, it would be desirable to have an RF amplifier that produces a predictable amount of IMD, in accordance with the theoretical 3:1 curve.
Existing techniques for compensating for excessive IMD are not adequate to this task. A first known technique is to decrease or attenuate the power level of the amplifier input signal. A decrease in input power causes a corresponding decrease in the output power of the amplifier, so that the third-order intermodulation and harmonic distortion is reduced to acceptable levels. The useful output power range over which the amplifier can be used is therefore limited, in order to avoid excessive IMD. This limitation in output power range is highly undesirable.
A second technique that has been used to compensate for excessive distortion is described in U.S. Pat. No. 5,712,593 (hereinafter, the ""593 patent). According to this technique, there is selected a power level of total distortion (including both harmonic and intermodulation distortion) that is acceptable in the output of the amplifier. An adjustable bias circuit is then employed to continually adjust the operating point of the amplifier, depending on the power of the distortion in the output of the amplifier, so that the power level of distortion that is acceptable is maintained. If the power of the distortion is too high, the operating point of the amplifier is raised to a point at which the amplifier operates more linearly and produces less distortion. Conversely, if the power of the distortion is lower than the acceptable amount, the operating point of the amplifier is lowered, thereby improving the efficiency of the amplifier but increasing the distortion produced by the amplifier.
Although the system of the ""593 patent successfully maintains the total distortion in the output of an amplifier at a predetermined power level, however, it does not accomplish the desired objective of causing the third-order IMD to increase at the 3:1 rate. Rather, in the system of the ""593 patent, the power level of the third-order IMD in the output of the amplifier remains at approximately the same level as the power of the input signal increases, because the amplifier bias voltage is continually adjusted to maintain the level of distortion constant. Thus, the technique of the ""593 patent does not fulfill the need for predictability in amplifier performance.
It is accordingly an object of the invention to provide an amplifier bias adjustment that maintains the theoretically-expected 3:1 third-order IMD slope for output power.
The invention is directed to a circuit for amplifying an input RF signal. The circuit comprises the following elements: (1) an RF amplifier circuit that amplifies the input RF signal and produces an output RF signal that comprises a third-order IMD product signal; (2) a power divider circuit that divides a signal output from the RF amplifier circuit into the output RF signal and a feedback signal; (3) a feedback circuit comprising an RF detector that detects the feedback signal and produces a DC voltage proportional to the power of the feedback signal that is used to bias the RF amplifier circuit. With this circuit, the third-order IMD product signal responds, over a predetermined range of power of the output RF signal, by increasing substantially three decibels in response to each one-decibel increase in the level of the input RF signal. Preferably, the feedback circuit further comprises a DC level shifter and a DC amplifier to amplify and shift the level of the DC voltage before it is used to bias the RF amplifier circuit.
In one embodiment, the power divider circuit produces the RF output signal and the feedback signal with equal strength. Preferably, though, the power divider circuit produces the RF output signal and the feedback signal with unequal strength. In a preferred embodiment, the feedback signal has a strength at least ten decibels less than the RF output signal. The power divider circuit may comprise a coupler with a ferrite core. Alternatively, it may comprise a resistor or any other circuitry that divides power. The RF amplifier circuit preferably comprises a monolithic gallium arsenide (GaAs) metal-semiconductor field-effect transistor (MESFET). The detector circuit preferably comprises a DC blocking capacitor, a detector diode, a DC bias inductor, a DC bias resistor, and an RF bypass capacitor.
The invention is also directed to a method for amplifying an input RF signal to produce an output RF signal that comprises a third-order IMD product signal, where the third-order IMD product signal responds over a predetermined range of power of the output RF signal by increasing substantially three decibels in response to each one-decibel increase in the level of the input RF signal. The method comprises the following steps: (1) amplifying the input RF signal in an amplifier to produce an amplified signal; (2) dividing the amplified signal into an output RF signal and a feedback signal; (3) generating a DC signal with a DC voltage proportional to the power of the feedback signal; and (4) biasing the amplifier based on the DC signal. Preferably, the method further comprises the steps of amplifying the DC signal and level-shifting the DC voltage before using it to bias the amplifier.
In one embodiment, the step of dividing produces the RF output signal and the feedback signal with equal strength. Preferably, though, this step produces the RF output signal and the feedback signal with unequal strength. In a preferred embodiment, the feedback signal has a strength at least ten decibels less than the RF output signal. The step of dividing may be performed with a circuit that comprises a coupler with a ferrite core. Alternatively, it may be performed with a circuit that comprises a resistor or any other circuitry that divides power. The amplifier preferably comprises a monolithic GaAs MESFET. The step of generating a DC signal is preferably performed with a circuit that comprises a DC blocking capacitor, a detector diode, a DC bias inductor, a DC bias resistor, and an RF bypass capacitor.